[CMake] Provide Findzstd module

This module is used to find the system zstd library. The imported
targets intentionally use the same name as the generate zstd config
CMake file so these can be used interchangeably.

Differential Revision: https://reviews.llvm.org/D134990

llvm/cmake/modules/Findzstd.cmake

@@ -0,0 +1,49 @@
+# Try to find the zstd library
+#
+# If successful, the following variables will be defined:
+# zstd_INCLUDE_DIR
+# zstd_LIBRARY
+# zstd_FOUND
+#
+# Additionally, one of the following import targets will be defined:
+# zstd::libzstd_shared
+# zstd::libzstd_static
+
+if(MSVC)
+  set(zstd_SHARED_LIBRARY_SUFFIX "\\${CMAKE_LINK_LIBRARY_SUFFIX}$")
+  set(zstd_STATIC_LIBRARY_SUFFIX "_static\\${CMAKE_STATIC_LIBRARY_SUFFIX}$")
+else()
+  set(zstd_SHARED_LIBRARY_SUFFIX "\\${CMAKE_SHARED_LIBRARY_SUFFIX}$")
+  set(zstd_STATIC_LIBRARY_SUFFIX "\\${CMAKE_STATIC_LIBRARY_SUFFIX}$")
+endif()
+
+find_path(zstd_INCLUDE_DIR NAMES zstd.h)
+find_library(zstd_LIBRARY NAMES zstd zstd_static)
+
+include(FindPackageHandleStandardArgs)
+find_package_handle_standard_args(
+    zstd DEFAULT_MSG
+    zstd_LIBRARY zstd_INCLUDE_DIR
+)
+
+if(zstd_FOUND)
+  if(zstd_LIBRARY MATCHES "${zstd_SHARED_LIBRARY_SUFFIX}$" AND
+     NOT TARGET zstd::libzstd_shared)
+    add_library(zstd::libzstd_shared SHARED IMPORTED)
+    set_target_properties(zstd::libzstd_shared PROPERTIES
+          INTERFACE_INCLUDE_DIRECTORIES "${zstd_INCLUDE_DIR}"
+          IMPORTED_LOCATION "${zstd_LIBRARY}")
+  endif()
+  if(zstd_LIBRARY MATCHES "${zstd_STATIC_LIBRARY_SUFFIX}$" AND
+     NOT TARGET zstd::libzstd_static)
+    add_library(zstd::libzstd_static STATIC IMPORTED)
+    set_target_properties(zstd::libzstd_static PROPERTIES
+        INTERFACE_INCLUDE_DIRECTORIES "${zstd_INCLUDE_DIR}"
+        IMPORTED_LOCATION "${zstd_LIBRARY}")
+  endif()
+endif()
+
+unset(zstd_SHARED_LIBRARY_SUFFIX)
+unset(zstd_STATIC_LIBRARY_SUFFIX)
+
+mark_as_advanced(zstd_INCLUDE_DIR zstd_LIBRARY)